How to Size a Water Heater

The right water heater size ensures you never run out of hot water during peak demand (morning showers, laundry, dishwashing). Too small means cold showers; too large wastes energy heating water you don't use. The key metric is first-hour rating (FHR), not just tank capacity.

Sizing Formula

Peak hour demand = Sum of hot water used by all fixtures during your busiest hour
FHR needed = Peak hour demand
Tank size = FHR − Recovery rate (gives minimum tank capacity)

Hot Water Usage by Fixture

• Shower (10 min): 20–25 gallons
• Bath: 30–40 gallons
• Dishwasher: 6–10 gallons per cycle
• Clothes washer (warm): 15–20 gallons per load
• Hand washing: 2–4 gallons per minute
• Kitchen sink: 3–5 gallons per use

Water Heater Types

• Storage tank (gas): $800–$1,500 installed. 30–75 gallon. 40–50 GPH recovery. Most common. 10–15 year lifespan.
• Storage tank (electric): $600–$1,200 installed. Slower recovery (20 GPH). Simpler installation. 10–15 year lifespan.
• Tankless (gas): $1,500–$3,500 installed. Unlimited hot water. 2–5 GPM flow rate. 20+ year lifespan. Requires adequate gas line.
• Tankless (electric): $1,000–$2,500 installed. Lower flow rate (2–3 GPM). Requires high amperage (100–150A). Best in warm climates.
• Heat pump (hybrid): $1,200–$2,500 installed. 2–3× more efficient than standard electric. Qualifies for tax credits. Needs warm ambient air. 10–15 year lifespan.

Sizing Guide by Household

• 1–2 people: 30–40 gallon tank, or small tankless (3.5 GPM)
• 3–4 people: 40–50 gallon tank, or mid tankless (5 GPM)
• 5–6 people: 50–75 gallon tank, or large tankless (8 GPM)
• 7+ people: 75+ gallon or two tanks, or commercial tankless

Frequently Asked Questions

What size do I need?

1–2 people: 30–40 gal. 3–4 people: 40–50 gal. 5+: 50–80 gal or tankless. Match FHR to peak demand.

How much hot water per shower?

20–25 gallons for a 10-minute shower at 2–2.5 GPM. Low-flow heads reduce to 15–20 gallons.

Tank vs tankless?

Tank: cheaper upfront, simpler. Tankless: unlimited hot water, more efficient, lasts longer, costs more upfront.

What is FHR?

First-hour rating: total hot water the heater delivers in one hour with a full tank. Found on the EnergyGuide label.

Gas vs electric?

Gas: faster recovery, cheaper to operate. Electric: simpler to install, heat pump models very efficient. Choose based on available utilities and energy costs.

Related Calculators

• Pipe Size Calculator — supply pipe sizing
• BTU Calculator — heating load
• Wire Size Calculator — electrical for water heaters

Water heater sizing is approximate. Actual requirements depend on usage patterns, inlet water temperature, and fixture flow rates. Consult a licensed plumber for installation and local code compliance. Gas water heaters require proper venting per IRC G2427.